Pervaporation is a technique of separation of liquid mixtures. The mixtures to be separated are conducted onto one side of membranes which are essentially impervious to liquids but will permit passage of vapors in a controlled manner. The other sides of the membranes are usually exposed to a vacuum and the vapors passing through the membranes are removed and condensed in a condenser. It is also possible to remove the vapors by means of a carrier gas and separate them from the carrier gas by condensation in the condenser. The membranes usually consist of plastic and permit passage of vapors at a rate which is larger the thinner the membranes are. The membranes, therefore, are very thin, flexible foils which are suitably mounted in special apparatus (pervaporators).
Liquid mixtures are generally separated by distillation. In numerous mixtures such as water-ethanol, chloroform-hexane, ethanol-cyclohexane, butanol-heptane, water-isopropanol, water-tetrahydrofuran, water-dioxane, methanol-acetone, methanol-benzene, ethanol-ethylacetate and methanol-methylacetate, one arrives at a concentration limit at which the mixture vaporizes azeotropically, that is, at which the mixture can no longer be separated. This problem can be alleviated or overcome by the addition of third compounds wherein the separation steps may be continued by extractive distillation, however, only with substantial efforts.
Pervaporation offers an alternative solution. In such a process one of the two components of a mixture is preferentially adsorbed by a membrane. A membrane is exposed at one side to a stream of the mixture and a vacuum is applied to the membrane at the opposite side so that the liquid compound adsorbed at one side vaporizes and passes through the membrane and is removed as vapor from its other side, thereby providing room for the adsorption by the membrane of additional liquid. Selection of suitable membranes permits the separation of components of a mixture, also of azeotropic mixtures.
Since this technique is relatively new, apparatus used presently are usually those developed for reverse osmosis processes. Reverse osmosis is a separation process based on different physical conditions such as high pressure differentials across the membrane, liquid states at both sides of, and during the flow through, the membrane, and no heat consumption upon permeation through the membrane.
For the utilization of foil-type membranes, apparatus with spirally wound membranes may be utilized. Supply chambers and permeation chambers are formed in this case by grid structures disposed between the membranes and consisting of plastic materials, stretch metal screens and similar structures which have a mesh structure through which the liquids can flow. The membranes are sealed in by cementing and disposed in tubular containers.
Another alternative arrangement utilizes frame-type modules which have flat membranes mounted, under tension, in the frame. The frame has transmission passages incorporated therein for conducting liquid to an adjacent chamber. Sealing between frames is obtained by O-rings or profiled seal rings. The design of the frame modules is such that there is a narrow chamber for the admission of the liquid mixture above the membrane. With its lower side the membrane is disposed on a grid structure with small openings or a ceramic support whose porosity permits removal of the permeates in collecting conduits (DE AS-28 02 780).
Such structures are assembled side-by-side with top and bottom end plates which are contained by threaded bolts so as to compress and seal the frame structures therebetween.
In accordance with the purpose of the modules, such arrangements are highly pressure resistant; they are built with large expenditures of materials. The housings, threaded bolts, flange members and pipe connectors are designed to withstand pressures of 40 to 100 bar--depending on the manufacturer. A reduction of the qualities of the materials and strength of the components to accommodate only the requirements for pervaporation of 1 bar pressure differential and vacuum operation, however, is not possible because the types of seals, and the duct structure for guiding the liquid by flow passages formed in profiled rubber sealed between adjacent plates, will not permit a substantial reduction of the thickness of the materials without disturbing the flow geometry.
The method of pervaporation, by its principle, also results in high heat consumption because of the vaporizing step at the membrane which may cool the solutions to such a degree that their flowing capability is greatly reduced whereby the pervaporation efficiency decreases exponentially.
The addition of heat as compensation for the heat consumption is necessary to prevent cooling of the mixture in the modules. Generally, the solution is reheated outside the module and supplied to a subsequent module. The length of a module, therefore, must depend on the acceptable or tolerated rate of cool-down of the mixture to be separated. However, since, on one hand, the degree of cooling depends directly on the membrane efficiency but, on the other hand, the pervaporation efficiency experiences rapid degradation with falling temperatures, acceptable conditions are achieved only with small modules.
It is obvious, however, that a compact arrangement of relatively large membranes within a relatively small number of modules which are relatively large in size is better and less expensive than the provision of the same membrane surface area in a large number of relatively small modules with heat exchangers arranged between consecutive modules especially since the capital expenditures for such apparatus are determined to a large extent by the end plate structures of each module (two for each module) provided with inlet and outlet nozzles.
With the large modules of the prior art, a small temperature differential between inlet and outlet flow may be achieved by pumping the solution through the modules at high flow rates, that is, at several times the speed necessary for the removal of the desired amount of the mixture component. Then, .DELTA.t may be reduced and the mixture can be reheated in a subsequent heat exchanger to make up for the heat taken from the mixture for the evaporation of part thereof through the membrane.
This, however, would require recirculation of the mixture whereby, for example, 90% of the mixture would be recirculated in each module and 10% would be passed on to the next module where it would again be subjected to recirculation with pervaporation of only a small fraction of the amount of mixture that needs to be circulated. Such an arrangement would require less capital investment than an arrangement with a large number of small modules but it results in relatively high operating costs since the circulating pumps, which are pumping ten times the amount of liquid at substantially increased speed and flow resistance, consume large amounts of energy.
U.S. Pat. Nos. 3,398,081; 3,520,803 and 3,695,444, all assigned to Ionics, Incorporated, Watertown, Mass., disclose pervaporation apparatus with a three-chamber module wherein heat is transmitted to the solution by means of integral chambers through which a hot liquid is conducted. In order to achieve good heat transfer, the hot liquid is conducted through each chamber in a tortuous path formed by divider walls, and the solution is conducted through the adjacent chambers through congruent tortuous paths which also have 180.degree. turns and consequently cause substantial turbulence in the solution conducted therethrough.
A major problem encountered in connection with the presently utilized modules is found in a relatively high vapor flow resistance at the vapor side of the modules. Caused by incomplete withdrawal of the mixture components passed through the membranes, an equilibrium status is developed which reduces or even destroys separation performance of a module. Considering the presently utilized modules it is noted that it is impossible to remove the amount of vapors at the secondary side of the membrane at a typical operating pressure of 5 m bar. With the given modules' designs at pervaporation rates of 25 kg/m.sup.2, the vapor velocities would--even considering the most favorable discharge designs--reach velocities of about 100 m/s simply which would result in high flow resistances in the modules' secondary side relatively small flow passages.
If, however, not properly removed from the modules' secondary side, the vapors may condense on the membranes and form membrane-wetting condensate films which initiate an osmotic process that causes an exchange of concentration through the membranes so that, on balance, there is no separation of the mixture admitted to a module. Even if most of the vapors can be removed through the discharge passages, the narrow draining passage arrangement results in insufficient vapor pressure relief which leads to capillary condensation in accordance with the principles of capillarity and surface tension.
Incomplete removal of the vapors therefore decreases the membrane performance with regard to the amount of permeate and separation quality. The prior art apparatus have an additional performance-reducing property which is caused by the flow pattern of the fluids in the various chambers of the apparatus. To facilitate explanation of this phenomenon, a typical pervaporation process in a particular module is analyzed: as far as permeation quality and permeation quantity is concerned, a pervaporation apparatus for the separation of a solution AB, which is to produce, for example, large amounts of the component A with only a small content of the component B, is highly dependent on the composition of the solution adjacent the membrane.
The following example makes this quite clear: A mixture of ethyl alcohol and water is to be separated as completely as possible by pervaporation through a cellulose triacetate membrane. Since the membrane preferably permits the passage of water accompanied only by a small amount of ethyl alcohol, the mixture may be so treated that, after extended processing, there is only practically pure ethyl alcohol at the admission side of the membrane after practically all the water has passed through the membrane and was removed as permeate. The permeate though is mixed with a small amount of ethyl alcohol since, practically, no absolutely perfect separation is possible.
The following table shows that the separation quality of the membranes depends, indeed depends greatly, on the composition of the solution directly adjacent the membrane.
______________________________________ Composition of the Raw Solution Composition of the Adjacent the Membrane Corresponding Permeates % Ethyl Alcohol % Water % Ethyl Alcohol % Water ______________________________________ 20 80 2 98 50 50 10 90 80 20 30 70 96 4 60 40 ______________________________________
The given values are typical and may be supplemented by numerous similar examples available from the literature.
Similarly, the quantity of permeate obtainable from a predetermined membrane area within a given time also is greatly dependent on the mixture ratio of the components to be separated. By way of a series of tests with solutions of different concentrations, the flow maxima and minima may be determined empirically.
Consequently, raw solution and permeate are in an equilibrium relationship which means that a change of the raw solution composition will automatically result in a corresponding change of the permeate composition until equilibrium is achieved. Also, if in a system in equilibrium, the equilibrium would be disturbed, for example, by the addition of water to the permeate, the permeate composition of subsequently produced permeates would contain less water until equilibrium is again achieved. Using the numbers in the given example the situation at the membranes would be as follows.
A mixture of 80% ethyl alcohol and 20% water reaches the membrane at the start at one side thereof. The corresponding permeate consists of 30% ethyl alcohol and 70% water. The flow of the mixture along the membrane at said one side loses water content, that is, the ethyl alcohol becomes more concentrated. As a result, somewhat downstream the mixture at said one side will consist, for example, of 90% ethyl alcohol and 10% water which will result in a permeate of 45% ethyl alcohol and 55% water.
It is clearly apparent that the amount of ethyl alcohol in the permeate is increasing at an undesirable rate, that is, water removal from the solution becomes less efficient. If, as in the prior art modules, the permeates are removed at one end of the permeate chamber, which end furthermore has only one or a small number of discharge openings, the permeates are not rapidly removed but violently and totally mixed, a permeate of relatively high concentration remains adjacent the membrane surface thereby reducing the efficiency and the separation capacity of the apparatus. This is especially true for the areas of the membrane adjacent the discharge openings where the mixtures are more concentrated anyway. The turbulent mixing of the permeates is especially disadvantageous when the flow direction of the raw mixture and that of the permeate at opposite sides of a membrane are the same. In that case--using the given example--permeates with a high water content are produced at the solution chamber inlet, which permeates, however, are conducted past permeates which are produced downstream and have a greater ethyl alcohol content, with which they mix and produce in the permeate a water concentration higher than that corresponding to the raw solution at the opposite side of the membrane. This, of course, results in a reduction of the separation quality in the affected areas of the membrane since the water content of the permeates may now be higher than in accordance with the equilibrium conditions.
It is, therefore, the object of the present invention to provide a pervaporation apparatus which can fulfill the requirements necessary for the pervaporation technique to succeed:
1. The membrane should have a large surface based on chamber volume. PA1 2. There should be a perfect seal between liquid and vapor chambers. PA1 3. The liquid supply stream should flow evenly across the membrane surface. PA1 4. The sensitive membrane should be adequately supported. PA1 5. Removal of vapors should not be obstructed by the membrane support means. PA1 6. The flow resistance on the secondary side of the membrane should be low so that a sufficiently low pressure can be maintained even during generation of large vapor volumes. PA1 7. The apparatus should have means which make it possible to replace the heat consumed by the pervaporation through the membranes.